Dual riser catalytic cracking process for making middle distillate and lower olefins

ABSTRACT

A fluidized catalytic cracking process and system that provide for the processing of hydrocarbon feedstocks to selectively produce a middle distillate boiling range product and lower olefins. The inventive process uses two riser reactors each having associated therewith a separator/stripper for separating the cracked product and cracking catalyst received from the respective riser reactor and a single regenerator for regenerating coked or spent cracking catalyst received from the separator/strippers. The two riser reactors, two separator/strippers and regenerator are operatively integrated to provide a process system for carrying out the process of the invention.

This application claims the benefit of U.S. Provisional Application No.61/503,209 filed Jun. 30, 2011, the entire disclosure of which is herebyincorporated by reference.

This invention relates to method and apparatus for the manufacture of amiddle distillate product and lower olefins from a hydrocarbon feedstockby the use of a dual riser catalytic system and process.

The fluidized catalytic cracking (FCC) of heavy hydrocarbons to producelower boiling hydrocarbon products, such as gasoline, has been aroundsince the 1940's. Typically, an FCC unit or process system includes asingle riser reactor, a catalyst separator and stripper, and aregenerator. An FCC feedstock is introduced into the riser reactor inwhich it is contacted with hot FCC catalyst from the regenerator. Themixture of FCC feedstock and FCC catalyst passes through the riserreactor and into the catalyst separator wherein the cracked product isseparated from the FCC catalyst. The separated cracked product passesfrom the catalyst separator to a downstream separation system and theseparated catalyst passes to the regenerator where the coke deposited onthe FCC catalyst during the cracking reaction is burned off the catalystto provide a regenerated catalyst. The resulting regenerated catalyst isused as the aforementioned hot FCC catalyst and is mixed with the FCCfeedstock that is introduced into the riser reactor.

Many processes and systems are designed so as to provide for a highconversion of the FCC feedstock to yield products having boilingtemperatures in the gasoline boiling range. But, some prior artprocesses provide for the preferential conversion of a hydrocarbonfeedstock to a middle distillate product and lower olefins. One suchprocess is disclosed in the US Patent Publication No. US 2006/0231461 ofMo et al. The process taught by Mo et al. includes the use of a riserreactor in combination with a dense bed reactor to process a gas oilfeedstock and a gasoline feedstock in a way to preferentially makemiddle distillate and lower olefins. The disclosed process includesregenerating a spent cracking catalyst and using the resultingregenerated catalyst in the dense bed reactor. Used regenerated catalystis passed from the dense bed reactor and introduced into the riserreactor wherein it is used in combination with regenerated catalyst inthe fluidized catalytic cracking of the gas oil feedstock.

Other publications disclose the use of a combination of riser reactorsor a combination of a dense fluid bed reactor with a riser reactor toprovide for recracking of a gasoline product from the cracking of gasoil. U.S. Pat. No. 3,928,172 to Davis, Jr. et al. discloses a number ofalternative fluid catalyst systems and processes that involve therecracking of cracked gasoline over a zeolite-containing catalyst. It isasserted by Davis that the zeolite catalyst is able to effect a degreeof octane improvement that was previously not possible with amorphoussilica-alumina catalysts.

One process and system disclosed by Davis uses a dense bed reactor witha single riser reactor arranged in a catalyst flow sequence such thatthe dense bed is placed between the regenerator and riser. Davis furtherdiscloses a hydrocarbon upgrading process that includes a first crackingzone in which gas oil is cracked and a second cracking zone in whichgasoline is cracked. The second cracking zone may include a dense bed.In another disclosure of Davis, gasoline is cracked within a dense bedreaction zone in which freshly regenerated catalyst is introduced.Catalyst from the dense bed reaction zone is then used for gas oilcracking in a riser cracking zone.

In the article published by the Chinese Journal of Chemical Engineering,16(3) 394-400 (2008), entitled “Alternative Processing Technology forConverting Vegetable Oils and Animal Fats to Clean Fuels and LightOlefins,” the authors Tian et al. disclose a catalytic cracking processthat utilizes two risers which share a common disengager andregenerator. Fresh feedstock is introduced into the first stage riser,and a recycle stream of gasoline or heavy oil, or both, is introducedinto the second stage riser. There is no disclosure, however, of the useof multiple catalyst separators or strippers nor is there any disclosureof the selective separation or stripping of catalyst taken from each ofthe two risers and the separate or selective recycle thereof.

One of the objects of this invention is to provide method and apparatusfor the preferential conversion of a hydrocarbon feedstock to a middledistillate product and lower olefins.

Accordingly, provided is a dual riser cracking process for making middledistillate and lower olefins, wherein said process comprises:catalytically cracking a first hydrocarbon feedstock within a firstriser reactor zone by contacting under first catalytic crackingconditions within said first riser reactor zone said first hydrocarbonfeedstock with a combination of a clean spent catalyst and a firstportion of a regenerated cracking catalyst to yield a first riserreactor product comprising a first cracked product and a coked spentcatalyst; catalytically cracking a second hydrocarbon feedstock within asecond riser reactor zone by contacting under second catalytic crackingconditions within said second riser reactor zone said second hydrocarbonfeedstock with a second portion of said regenerated cracking catalyst toyield a second riser reactor product comprising a second cracked productand said clean spent catalyst; passing said first riser reactor productto a first separator/stripper providing means for separating said firstriser reactor product into a separated first cracked product and aseparated coked spent catalyst; passing said second riser reactorproduct to a second separator/stripper providing means for separatingsaid second riser reactor product into a separated cracked secondcracked product and a separated clean spent catalyst; using at least aportion of said separated clean spent catalyst as said clean spentcatalyst of said combination; and passing said separated coked spentcatalyst and a remaining portion of said separated clean spent catalystto a regenerator that defines a regeneration zone and provides means forregenerating said separated coked spent catalyst and said remainingportion of said separated clean spent catalyst to yield said regeneratedcracking catalyst.

FIG. 1 is a process flow schematic illustrating certain aspects of oneembodiment of the inventive process.

The invention includes process and apparatus that provide for theprocessing of hydrocarbon feedstocks to selectively or preferentiallyproduce a middle distillate boiling range product and lower olefins. Theinventive process uses two riser reactors each having associatedtherewith a separator/stripper for separating the cracked product andcracking catalyst received from the respective riser reactor and asingle regenerator for regenerating coked or spent cracking catalystreceived from the separator/strippers. The two riser reactors, twoseparator/strippers and regenerator are operatively integrated toprovide a process system for carrying out the process of the invention.

In the inventive process, a first hydrocarbon feedstock is introducedinto the bottom of a first riser reactor zone that is defined by a firstriser reactor. Hot cracking catalyst (e.g., the first portion of theregenerated cracking catalyst, as defined below, and, optionally, atleast a portion of, or, alternatively, a remaining portion of, theseparated clean spent catalyst, as they are defined below) is alsointroduced into the first riser reactor zone, wherein it is mixed andcontacted with the first hydrocarbon feedstock under suitable firstcatalytic cracking conditions to provide for catalytically cracking thefirst hydrocarbon feedstock.

A second hydrocarbon feedstock is introduced into the bottom of thesecond riser reactor zone that is defined by a second riser reactor.Regenerated catalyst (e.g., the second portion of regenerated crackingcatalyst, as defined below) is also introduced into the second riserreactor zone, wherein it is mixed and contacted with the secondhydrocarbon feedstock under suitable second catalytic crackingconditions to provide for catalytically cracking the second hydrocarbonfeedstock.

The fresh catalytic cracking catalyst used in the inventive process andcirculated within the process system can be any suitable crackingcatalyst known in the art to have cracking activity under the catalyticcracking conditions contemplated by the invention. Preferred catalyticcracking catalysts for use in the inventive process include fluidizablecracking catalysts comprised of a molecular sieve having crackingactivity dispersed in a porous, inorganic refractory oxide matrix orbinder.

The term “molecular sieve” as used herein refers to any material capableof separating atoms or molecules based on their respective dimensions.Molecular sieves suitable for use as a component of the crackingcatalyst include pillared clays, delaminated clays, and crystallinealuminosilicates. Normally, it is preferred to use a cracking catalystthat contains a crystalline aluminosilicate. Examples of suchaluminosilicates include Y zeolites, ultrastable Y zeolites, X zeolites,zeolite beta, zeolite L, offretite, mordenite, faujasite, and zeoliteomega. The preferred crystalline aluminosilicates for use in thecracking catalyst are X and Y zeolites with Y zeolites being the mostpreferred.

U.S. Pat. No. 3,130,007, the disclosure of which is hereby incorporatedby reference in its entirety, describes Y-type zeolites having anoverall silica-to-alumina mole ratio between about 3.0 and about 6.0,with a typical Y zeolite having an overall silica-to-alumina mole ratioof about 5.0. It is also known that Y-type zeolites can be produced,normally by dealumination, having an overall silica-to-alumina moleratio above about 6.0. Thus, for purposes of this invention, a Y zeoliteis one having the characteristic crystal structure of a Y zeolite, asindicated by the essential X-ray powder diffraction pattern of Yzeolite, and an overall silica-to-alumina mole ratio above 3.0, andincludes Y-type zeolites having an overall silica-to-alumina mole ratioabove about 6.0.

The stability and/or acidity of a zeolite used as a component of thecracking catalyst may be increased by exchanging the zeolite withhydrogen ions, ammonium ions, polyvalent metal cations, such as rareearth-containing cations, magnesium cations or calcium cations, or acombination of hydrogen ions, ammonium ions and polyvalent metalcations, thereby lowering the sodium content until it is less than about0.8 weight percent, preferably less than about 0.5 weight percent andmost preferably less than about 0.3 weight percent, calculated as Na₂O.Methods of carrying out the ion exchange are well known in the art.

The zeolite or other molecular sieve component of the cracking catalystis combined with a porous, inorganic refractory oxide matrix or binderto form a finished catalyst prior to use. The refractory oxide componentin the finished catalyst may be silica-alumina, silica, alumina, naturalor synthetic clays, pillared or delaminated clays, mixtures of one ormore of these components and the like. Preferably, the inorganicrefractory oxide matrix will comprise a mixture of silica-alumina and aclay such as kaolin, hectorite, sepiolite and attapulgite.

A preferred finished catalyst will typically contain between about 5weight percent to about 40 weight percent zeolite or other molecularsieve and greater than about 20 weight percent inorganic, refractoryoxide. In general, the finished catalyst may contain between about 10 toabout 35 weight percent zeolite or other molecular sieve, between about10 to about 30 weight percent inorganic, refractory oxide, and betweenabout 30 to about 70 weight percent clay.

The crystalline aluminosilicate or other molecular sieve component ofthe cracking catalyst may be combined with the porous, inorganicrefractory oxide component or a precursor thereof by any suitabletechnique known in the art including mixing, mulling, blending orhomogenization. Examples of precursors that may be used include alumina,alumina sols, silica sols, zirconia, alumina hydrogels, polyoxycationsof aluminum and zirconium, and peptized alumina.

In a preferred method of preparing the cracking catalyst, the zeolite iscombined with an alumino-silicate gel or sol or other inorganic,refractory oxide component, and the resultant mixture is spray dried toproduce finished catalyst particles normally ranging in diameter betweenabout 40 and about 80 microns. If desired, however, the zeolite or othermolecular sieve may be mulled or otherwise mixed with the refractoryoxide component or precursor thereof, extruded and then ground into thedesired particle size range. Normally, the finished catalyst will havean average bulk density between about 0.30 and about 0.90 gram per cubiccentimeter and a pore volume between about 0.10 and about 0.90 cubiccentimeter per gram.

The first hydrocarbon feedstock may be any suitable hydrocarbonfeedstock that is chargeable to a fluidized catalytic cracking unit orthat will result in providing a particularly desired product mix. In onepreferred embodiment of the inventive process, the first hydrocarbonfeedstock is a gas oil. Hydrocarbon mixtures boiling in the range offrom 345° C. (653° F.) to 760° C. (1400° F.) can suitably be used as thefirst hydrocarbon feedstock of the invention. Examples of the types ofrefinery feed streams that can make suitable gas oil feedstocks includevacuum gas oils, coker gas oils, straight-run residues, thermallycracked oils and other hydrocarbon streams.

The catalytic cracking conditions can be defined by such parameters asthe average residence time of the hydrocarbons in a particular riserreactor, the catalyst-to-oil ratio, and the riser reactor temperature.

The first catalytic cracking conditions at which the first riser reactorzone is operated can include an average residence time of thehydrocarbons (e.g., first hydrocarbon feedstock) in the first riserreactor zone that is generally in the range of upwardly to about 5 to 10seconds, but, usually, it is in the range of from 0.1 to 5 seconds. Theweight ratio of catalyst (e.g., the first portion of regeneratedcracking catalyst, as defined below, and, optionally, at least a portionof, or, alternatively, a remaining portion of, the separated clean spentcatalyst, as defined below) to first hydrocarbon feedstock (i.e., thecatalyst/oil ratio) introduced into the first reactor zone generally canbe in the range of from about 2 to about 100 and even as high as 150.More typically, the catalyst-to-oil ratio can be in the range of from 5to 100. The temperature in the first riser reactor zone generally can bein the range of from about 400° C. (752° F.) to about 600° C. (1112°F.). More typically, the first riser reactor temperature can be in therange of from 450° C. (842° F.) to 550° C. (1022° F.).

The second hydrocarbon feedstock may be any suitable hydrocarbonfeedstock that is chargeable to a fluidized catalytic cracking unit orthat will provide the particularly desired product mix. In a preferredembodiment of the inventive process, the second hydrocarbon feedstockinclude hydrocarbon mixtures boiling in the naphtha or gasoline boilingtemperature range. Generally, gasoline feedstocks comprise hydrocarbonsboiling in the temperature range of from about 32° C. (90° F.) to about204° C. (400° F.). Examples of refinery streams that may be used as thenaphtha or gasoline feedstock of the inventive process include straightrun gasoline, straight run naphtha, catalytically cracked gasoline, andcoker naphtha.

The second catalytic cracking conditions at which the second riserreactor zone is operated can include an average residence time of thehydrocarbons (e.g., second hydrocarbon feedstock) in the second riserreactor zone generally in the range upwardly to about 20 seconds, butusually the average residence time is in the range of from 0.1 to 10seconds. The weight ratio of catalyst (e.g., the second portion ofregenerated cracking catalyst, as defined below) to second hydrocarbonfeedstock (i.e., the catalyst/oil ratio) can generally be in the rangeof from about 2 to about 100 and even as high as 150. More typically,the catalyst-to-oil ratio can be in the range of from 5 to 100. Thetemperature in the second riser reactor zone generally can be in therange of from about 482° C. (900° F.) to about 871° C. (1600° F.). Moretypically, the second riser reactor zone generally can be in the rangeof from 538° C. (1000° F.) to 732° C. (1350° F.).

The hot cracking catalyst that is introduced into the first riserreactor zone along with the first hydrocarbon feedstock includes a firstportion of regenerated cracking catalyst taken from the regenerator. Inanother embodiment of the inventive process, at least a portion of theseparated clean spent catalyst is introduced into the first riserreactor zone along with the first hydrocarbon feedstock and the firstportion of the regenerated cracking catalyst. The remaining portion ofseparated clean spent catalyst, which is the portion that is notintroduced into the first riser reactor zone, is passed and introducedinto the regenerator. In an alternative embodiment of the invention, atleast a portion of the separated clean spent catalyst is, instead,passed and introduced into the regenerator; and, then the remainingportion of the separated clean spent catalyst, which is the portion thatis not introduced into the regenerator, is passed and introduced intothe first riser reactor zone.

The clean spent catalyst is referred to herein as being “clean” becauseit is derived from the product of the second riser reactor zone that isdefined by a second riser reactor. The second riser reactor zone of theprocess is operated under suitable second catalytic cracking conditionsthat are more severe than the reaction conditions under which the firstriser reactor zone is operated. The second hydrocarbon feedstock chargedto the second riser reactor zone is, preferably, a lighter feedstockthan the first hydrocarbon feedstock charged to the first riser reactorzone, thus resulting in less coke yield.

Due to the cracking of a lighter feedstock, the used cracking catalystyielded from the product of the second riser reactor zone has a lowerconcentration of coke than the spent or coked spent catalyst yieldedfrom the product of the first riser reactor zone. Thus, the usedcracking catalyst from the second riser reactor product is referred toherein as being “clean” for the purpose of distinguishing it from thespent or coked spent catalyst yielded from the first riser reactorproduct.

Yielded from the first riser reactor zone is a first riser reactorproduct that comprises a first cracked product and a coked spentcatalyst. The first riser reactor product is passed to the firstseparator/stripper associated with the first riser reactor. The firstseparator/stripper provides means for separating the first riser reactorproduct into a separated first cracked product and a separated cokedspent catalyst.

The separated coked spent catalyst has a coke content, generally, in therange of from about 0.5 to about 5 weight percent (wt. %), based on thetotal weight of the catalyst and the carbon. More typically, the cokecontent on the separated coked spent catalyst is in the range of from orabout 0.5 wt. % to or about 1.5 wt. %.

A second riser reactor product is yielded from the second riser reactorzone and comprises a second cracked product and a clean spent catalyst.This second riser reactor product is passed to the secondseparator/stripper that is associated with the second riser reactor. Thesecond separator/stripper provides means for separating the second riserreactor product into a separated second cracked product and a separatedclean spent catalyst.

The separated clean spent catalyst has a coke content that is typicallylower than the coke content of the separated coked spent catalyst.Generally, the coke content of the separated clean spent catalyst is inthe range of from about 0.1 to about 1 weight percent (wt. %), based onthe total weight of the catalyst and the carbon. More typically, thecoke content on the separated clean spent catalyst is in the range offrom or about 0.1 wt. % to or about 0.6 wt. %.

Each separator/stripper of the inventive fluidized catalytic crackingprocess system includes a vessel that defines a separation zone and astripping zone. Within the separation zone there may be one or morecyclones that define one or more cyclone separation zones. The cyclonesmay be operated in series flow or in parallel flow and they providemeans for receiving a riser reactor effluent and for separating spentcatalyst and catalytically cracked vaporous hydrocarbons of the riserreactor effluent. The separated vaporous hydrocarbon product exits thecyclones and the separator/stripper apparatus to pass downstream forfurther processing such as with a main fractionator of the fluidizedcatalytic cracking unit. The separated spent catalyst passes from thecyclones through diplegs into the stripping zone or section of theseparator/stripper apparatus. The separated spent catalyst is strippedof hydrocarbons within the stripping zone, typically, by the use ofstripping steam that is introduced into the stripping section of theseparator/stripper apparatus. Stripped catalyst is removed from thestripping section of the stripping section of the separator/stripperapparatus by way of a catalyst standpipe conduit.

The first riser reactor product is received into first separation meansof the first separator/stripper which defines a first separation zone.First separation means provides for separating the first riser reactorproduct into a separated first cracked product and a separated cokedspent catalyst.

The second riser reactor product is received into second separationmeans of the second separator/stripper which defines the secondseparation zone. Second separation means provides for separating thesecond riser reactor product into a separated second cracked product anda separated clean spent catalyst.

Any suitable means known in the art may be used as either firstseparation means or second separation means, but, typically, andpreferably, such means include cyclone separators that utilizecentrifugal flow and gravity to provide for the separation ofhydrocarbon gases and catalyst particles. Many of the various types ordesigns of suitable cyclone separators and their uses are known to thoseskilled in the art.

The separated first cracked product and separated second cracked productthat are respectively yielded from the first separator/stripper andsecond separator/stripper pass to the downstream where they may furtherbe processed. It is preferred for the separated first cracked productand the separated second cracked product to pass either separately or incombination to one or more fractionators, but, typically, a combinedstream is passed to a main fractionator.

The main fractionator defines a fractionation zone and provides meansfor separating the separated first cracked product or the separatedsecond cracked product, or a combination of both, into one or moreproduct streams including a naphtha product stream. Other productstreams may include a lower olefins stream, a cracked gasoline stream,and a cracked gas oil stream.

The first separator/stripper further defines a first stripping zonegenerally contained within the bottom section of the firstseparator/stripper. In the operation of the first separator/stripper,the separated coked spent catalyst falls from first separation meansinto the first stripping zone, wherein it is stripped of hydrocarbons.Any suitable stripping fluid may be used to strip the hydrocarbons fromthe separated coked spent catalyst, but, the preferred stripping fluidis steam, which may, in general, be introduced into the bottom of thefirst stripping zone or section of the first separator/stripper. Thestripped hydrocarbons will pass to downstream along with the separatedfirst cracked product for further processing and the separated cokedspent catalyst passes from the first stripping zone to be introducedinto the regeneration zone of the regenerator.

The second separator/stripper further defines a second stripping zonegenerally contained within the bottom section of the secondseparator/stripper. In the operation of the second separator/stripper,the separated clean spent catalyst falls from second separation meansinto the second stripping zone, wherein it is stripped of hydrocarbons.As with the first separator/stripper, any suitable stripping fluid maybe used to strip the hydrocarbons from the separated clean spentcatalyst with the preferred stripping fluid being steam. The strippedhydrocarbons will pass to downstream along with the separated secondcracked product for further processing.

The separated clean spent catalyst passes from the second stripping zoneand at least a portion of the separated clean spent catalyst includingup to the entire flow of the separated clean spent catalyst from thesecond separator/stripper is introduced into the regeneration zone ofthe regenerator. In another embodiment of the invention, the remainingportion of the separated clean spent catalyst that is not introducedinto the regenerator is further passed to and introduced in combinationor along with a first portion of the regenerated cracking catalyst intothe riser reactor zone of the first riser reactor. In still anotherembodiment of the inventive process, at least a portion of the separatedclean spent catalyst, instead, is introduced, in combination or alongwith a first portion of a regenerated cracking catalyst, into the firstriser reactor zone of the first riser reactor wherein the catalyst iscontacted with the first hydrocarbon feedstock. A remaining portion ofthe separated clean spent catalyst, which is that portion of theseparated clean spent catalyst that is not passed to and introduced intothe first riser reactor zone, is introduced into the regeneration zoneof the regenerator.

The regenerator of the inventive process or system provides means forregenerating the separated coked spent catalyst and the at least aportion or the remaining portion of the separated clean spent catalystto yield regenerated cracking catalyst. The regenerator defines aregeneration zone into which the separated coked spent catalyst and theat least a portion, or the remaining portion, of the separated cleanspent catalyst are introduced and wherein deposited carbon is burned toprovide the regenerated cracking catalyst having a reduced carboncontent. The regenerator, typically, is a vertical vessel of anysuitable configuration that defines the regeneration zone and whereinthe separated coked spent catalyst and the remaining portion, or atleast a portion, of the separated clean spent catalyst is maintained asa fluidized bed by the upward passage of an oxygen-containingregeneration gas, such as air.

The regeneration temperature within the regeneration zone is, ingeneral, maintained in the range of from about 621° C. (1150° F.) to760° C. (1400° F.), and more typically, in the range of from 677° C.(1250° F.) to 715° C. (1320° F.).

The pressure within the regeneration zone typically is in the range offrom about atmospheric to about 345 kPa (50 psig), and, preferably, fromabout 34 to 345 kPA (5 to 50 psig).

The residence time of the separated coked spent catalyst and the atleast a portion, or the remaining portion, of the separated clean spentcatalyst within the regeneration zone is in the range of from about 1 toabout 6 minutes, and, typically, from or about 2 to or about 4 minutes.

The coke content on the regenerated cracking catalyst is less than thecoke content on the separated coked spent catalyst and the at least aportion, or remaining portion, of the separated clean spent catalystthat are introduced into the regeneration zone of the regenerator. Thecoke content of the regenerated cracking catalyst will, thus, generallybe in the range of from or about 0.01 wt. % to or about 0.5 wt. %. It ispreferred for the coke concentration on the regenerated crackingcatalyst to be less than 0.1 wt. % and, it will preferably be in therange of from 0.01 wt. % to 0.1 wt. %.

The regenerated cracking catalyst yielded from the regenerator is usedas a hot cracking catalyst that is introduced into the first riserreactor zone and the second riser reactor zone for contacting with therespective hydrocarbon feedstocks. Thus, a first portion of theregenerated cracking catalyst is introduced into the first riser reactorzone wherein it is contacted with the first hydrocarbon feedstock, and asecond portion of the regenerated cracking catalyst is introduced intothe second riser reactor zone wherein it is contacted with the secondhydrocarbon feedstock.

Now referring to FIG. 1 which presents a process flow schematicrepresenting certain aspects of the inventive process 10. In process 10,a first hydrocarbon feedstock, which preferably is a gas oil feedstock,is passed by way of conduit 12 and introduced into first riser reactorzone 14 that is defined by first riser reactor 16.

First riser reactor 16 is an elongated conduit that extends vertically.The first hydrocarbon feedstock is introduced into first riser reactorzone 14 at or near the bottom of first riser reactor 16, wherein it ismixed or contacted with hot catalyst. The mixture of first hydrocarbonfeedstock and hot catalyst passes through first riser reactor zone 14,which is operated under suitable first catalytic cracking conditions soas to provide a first riser reactor product.

The first riser reactor product, which comprises a first cracked productand a coked spent catalyst, is yielded from first riser reactor zone 14and passes by way of conduit 18 from outlet 20 of first riser reactor 16to be introduced into first separator/stripper 22 through inlet 24.

The sources of hot catalyst introduced into first riser reactor zone 14include a first portion of the regenerated cracking catalyst taken fromregenerator 26, and, in an alternative embodiment of the invention, atleast a portion, or, in another alternative embodiment of the invention,a remaining portion, of the separated clean spent catalyst taken fromsecond separator/stripper 30. Thus, a first portion of the regeneratedcracking catalyst passes from regenerator 26 by way of conduit 32 and isintroduced into first riser reactor zone 14. Or, a first portion of theregenerated cracking catalyst in combination with at least a portion ofthe separated clean spent catalyst is introduced into first riserreactor zone 14. Or, a first portion of the regenerated crackingcatalyst in combination with a remaining portion of the separated cleanspent catalyst is introduced into first riser reactor zone 14. The atleast a portion of the separated clean spent catalyst is introduced intofirst riser reactor zone 14 by way of conduit 34. The remaining portionof the separated clean spent catalyst, likewise, is introduced intofirst riser reactor zone 14 by way of conduit 34.

First separator/stripper 22 includes and defines first separation zone36 and first stripping zone 38. One or more cyclones (not shown) may beincluded within first separation zone 36 to provide first separationmeans for separating the first riser reactor product into a separatedfirst cracked product and a separated coked spent catalyst. Providedwithin first stripping zone 36 are baffles or trays (not shown) thatprovide for enhanced contact between the falling catalyst and astripping fluid, such as steam, that is introduced into the firststripping zone by way of conduit 40, so as to assist in the stripping ofthe hydrocarbons from the falling catalyst. The separated coked spentcatalyst passes from first stripping zone 38 by way of conduit 44 to beintroduced into regeneration zone 46 of regenerator 26.

Regenerator 26 defines regeneration zone 46 and provides means forregenerating the separated coked spent catalyst by contacting theseparated coked spent catalyst with an oxygen-containing gas, such asair, under carbon burning conditions to remove carbon therefrom. Theoxygen-containing gas is introduced into regeneration zone 46 throughconduit and the combustion gases pass from regeneration zone 46 by wayof conduit 50 to downstream for further handling or processing.

Second riser reactor 54 is an elongated conduit that extends vertically.The second hydrocarbon feedstock, which is preferably a naphtha orgasoline feedstock, is introduced by way of conduit 55 into second riserreactor zone 56 at or near the bottom of second riser reactor 54,wherein it is mixed or contacted with a second portion of regeneratedcracking catalyst. The second portion of regenerated cracking catalystpasses from regenerator 26 by way of conduit 58 and is introduced intosecond riser reactor zone 56 by way of conduit The mixture of secondhydrocarbon feedstock and the second portion of regenerated crackingcatalyst passes through second riser reactor zone 56, which is operatedunder suitable second catalytic cracking conditions so as to provide asecond riser reactor product.

The second riser reactor product, which comprises a second crackedproduct and a clean spent catalyst, is yielded from second riser reactorzone 56 and passes by way of conduit 60 from outlet 64 of second riserreactor 54 to be introduced into second separator/stripper 30 throughinlet 66.

Second separator/stripper 30 provides means for receiving the secondriser reactor product by way of conduit 60 and through inlet 66. Secondseparator/stripper 30 includes and defines second separation zone 68 andsecond stripping zone 70. One or more cyclones (not shown) may beincluded within second separation zone 68 to provide second separationmeans for separating the second riser reactor product into a separatedsecond cracked product and a separated clean spent catalyst. Providedwithin second stripping zone 70 are baffles or trays (not shown) thatprovide for enhanced contact between the falling catalyst and astripping fluid, such as steam, that is introduced into the secondstripping zone by way of conduit 72, so as to assist in the stripping ofthe hydrocarbons from the falling catalyst.

The separated clean spent catalyst, which may be introduced intoregeneration zone 46 of regenerator 26, passes from second strippingzone 70 by way of conduit 74. At least a portion of the separated cleanspent catalyst up to and including the entire portion or flow of theseparated clean spent catalyst can be introduced into regeneration zone46 by way of conduit 76. Then, the remaining portion of the separatedclean spent catalyst that is not introduced into regenerator 26 passesby way of conduit 34 for introduction into first riser reactor zone 14.Or, in another alternative embodiment of the invention, a remainingportion of the separated clean spent catalyst, which is that portion ofthe separated clean spent catalyst not passed to first riser reactorzone 14 through conduit 34, can be introduced into regeneration zone 46by way of conduit 76. In this embodiment, at least a portion of theseparated clean spent catalyst is passed by way of conduit 34 to beintroduced into first riser reactor zone 14.

Regenerator 26 further provides means for regenerating the separatedcoked spent catalyst and the separated clean spent catalyst that arecharged to it. Thus, regenerator 26 defines a regeneration zone 46 andprovides means for regenerating separated coked spent catalyst and aremaining portion, or at least a portion, of separated clean spentcatalyst to yield regenerated cracking catalyst.

A separated first cracked product passes from first separator/stripper22 by way of conduit 80. A separated second cracked product passes fromsecond separator/stripper 30 by way of conduit 82. Either the separatedfirst cracked product or separated second cracked product, or acombination of the two streams, may be passed to a main fractionationcolumn or system (not shown). The main fractionation column or systemmay be any separation system know to those skilled in the art forrecovering and separating cracked product streams into various FCCproducts, such as, for example, cracked gas, cracked gasoline, crackedgas oils and cycle oil that respectively pass from main fractionationcolumn. A main fractionation system may include such systems asabsorbers and strippers, fractionators, compressors and separators orany combination of known systems for providing recovery and separationof the cracked products that may make up the separated first crackedproduct or the separated second cracked product, or both. In a preferredembodiment of the process, a combination of the separated first crackedproduct and the separated second cracked product passes to a mainfractionation column which provides means for their separation into oneor more product streams, as indicated above, including the crackedgasoline or a naphtha product stream.

In one embodiment of the inventive process, at least a portion of thecracked gasoline or naphtha product stream passing from the mainfractionation system is recycled and utilized as the second hydrocarbonfeedstock introduced into second riser reactor zone 34 by way of conduit44.

1. A dual riser catalytic cracking process for making middle distillateand lower olefins, wherein said process comprises: catalyticallycracking a first hydrocarbon feedstock within a first riser reactor zoneby contacting under first catalytic cracking conditions within saidfirst riser reactor zone said first hydrocarbon feedstock with acombination of a clean spent catalyst and a first portion of aregenerated cracking catalyst to yield a first riser reactor productcomprising a first cracked product and a coked spent catalyst;catalytically cracking a second hydrocarbon feedstock within a secondriser reactor zone by contacting under second catalytic crackingconditions within said second riser reactor zone said second hydrocarbonfeedstock with a second portion of said regenerated cracking catalyst toyield a second riser reactor product comprising a second cracked productand said clean spent catalyst; passing said first riser reactor productto a first separator/stripper providing means for separating said firstriser reactor product into a separated first cracked product and aseparated coked spent catalyst; passing said second riser reactorproduct to a second separator/stripper providing means for separatingsaid second riser reactor product into a separated cracked secondcracked product and a separated clean spent catalyst; using at least aportion of said separated clean spent catalyst as said clean spentcatalyst of said combination; and passing said separated coked spentcatalyst and a remaining portion of said separated clean spent catalystto a regenerator that defines a regeneration zone and provides means forregenerating said separated coked spent catalyst and said remainingportion of said separated clean spent catalyst to yield said regeneratedcracking catalyst.
 2. A process as recited in claim 1, which furthercomprises: passing said separated first cracked product and saidseparated second cracked product to a fractionator defining afractionation zone and providing fractionation means for separatingeither said separated first cracked product or said separated secondcracked product, or both, into one or more product streams including anaphtha product stream.
 3. A process as recited in claim 2, whichfurther comprises: using at least a portion of said naphtha productstream as at least a portion of said second hydrocarbon feedstock.
 4. Aprocess as recited in claim 3, wherein said first separator/stripperdefines a first separation zone that includes first separation means forseparating said first riser reactor product into said separated firstcracked product and said separated coked spent catalyst, and whereinsaid first separator/stripper further defines a first stripping zone,and wherein within said first stripping zone said separated coked spentcatalyst is stripped of hydrocarbons.
 5. A process as recited in claim4, wherein said second separator/stripper defines a second separationzone that includes second separation means for separating said secondriser reactor product into said separated second cracked product andsaid separated clean spent catalyst, and wherein said secondseparator/stripper further defines a second stripping zone, and whereinwithin said second stripping zone said separated clean spent catalyst isstripped of hydrocarbons.
 6. A dual riser cracking process for makingmiddle distillate and lower olefins, wherein said process comprises:catalytically cracking a first hydrocarbon feedstock within a firstriser reactor zone by contacting under first catalytic crackingconditions within said first riser reactor zone said first hydrocarbonfeedstock with a combination of a clean spent catalyst and a firstportion of a regenerated cracking catalyst to yield a first riserreactor product comprising a first cracked product and a coked spentcatalyst; catalytically cracking a second hydrocarbon feedstock within asecond riser reactor zone by contacting under second catalytic crackingconditions within said second riser reactor zone said second hydrocarbonfeedstock with a second portion of said regenerated cracking catalyst toyield a second riser reactor product comprising a second cracked productand said clean spent catalyst; passing said first riser reactor productto a first separator/stripper providing means for separating said firstriser reactor product into a separated first cracked product and aseparated coked spent catalyst; passing said second riser reactorproduct to a second separator/stripper providing means for separatingsaid second riser reactor product into a separated cracked secondcracked product and a separated clean spent catalyst; and passing saidseparated coked spent catalyst to a regenerator and passing saidseparated clean spent catalyst to said regenerator, wherein saidregenerator defines a regeneration zone and provides means forregenerating said separated coked spent catalyst and said separatedclean spent catalyst to yield said regenerated cracking catalyst.